Remove model from WidgetManager._model on comm:close.

IPython/html/static/widgets/js/manager.js

@@ -197,9 +197,13 @@
 
         WidgetManager.prototype._handle_comm_open = function (comm, msg) {
             // Handle when a comm is opened.
+            var that = this;
             var model_id = comm.comm_id;
             var widget_type_name = msg.content.target_name;
             var widget_model = new WidgetManager._model_types[widget_type_name](this, model_id, comm);
+            widget_model.on('comm:close', function () {
+              delete that._models[model_id];
+            });
             this._models[model_id] = widget_model;
         };